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5.2 CHAPTER FIVE

TYPES OF AERA TION/AIR STRIPPING EQUIPMENT

Structures or equipment for aeration or air stripping may be classified into four general
categories: waterfall aerators, diffusion or bubble aerators, mechanical aerators, and pres-
sure aerators.

Waterfall Aeration--Air Stripping
The waterfall type of aeration equipment utilizes air stripping and accomplishes gas trans-
fer by causing water to break into drops or thin films, increasing the area of water ex-
posed to air. The more common types are

• Spray aerators
• Multiple-tray aerators
• Cascade aerators
• Cone aerators
• Packed columns

Spray Aerators. Spray aerators direct water upward, vertically or at an inclined angle
in a manner that causes water to be broken into small drops. Installations commonly con-
sist of fixed nozzles or a pipe grid located over an open-top tank.
Spray aerators are usually efficient with respect to gas transfer such as carbon diox-
ide removal or oxygen addition. However, they require a large installation area, are dif-
ficult to house, and pose operating problems during freezing weather. To be most effec-
tive, spray aerators need a supplemental air exchange to minimize the buildup of
contaminants in the air phase.

Multiple.Tray Aerators. Multiple-tray aerators consist of a series of trays equipped with
slatted, perforated, or wire-mesh bottoms. Water is distributed at the top, cascades from
each tray, and is collected in a basin at the base. It is important to have even distribution
of water from the trays to obtain optimum unit efficiency. Coarse media such as coke,
stone, or ceramic balls ranging in size from 2 to 6 in. (5 to 15 cm) are used in many tray
aerators to improve the efficiency of gas exchange and to take advantage of the catalytic
effects of deposited manganese oxides.
A type of multiple-tray aerator, the crossflow tower, has been extensively used in wa-
ter cooling applications. Water is allowed to fall over the tray area while air is either
forced or induced to flow across the slats, perpendicular to the water path. Tray aerators
are analogous to cooling towers, and the problems encountered in design and operation
are similar. Tray aerators must be provided with adequate ventilation. If they are placed
in a poorly ventilated building, performance will be impaired by contamination of inlet
air with the compounds being removed.
Artificial ventilation is provided to some types of tray aerators by supplying air from
a blower at the bottom of the enclosure. These aerators exhibit excellent oxygen absorp-
tion and carbon dioxide, ammonia, and hydrogen sulfide removal.

Cascade Aerators. With cascade aerators, increases in exposure time and area-volume
ratio are obtained by allowing water to flow downward over a series of steps or baffles.
The simplest cascade aerator is a concrete step structure that allows water to fall in thin